The present invention relates to a vibration preventing apparatus for supporting a vibrating body in a vibration-proof configuration and, more particularly, to a vibration preventing apparatus of the type which absorbs vibrations by using the elasticity of an elastic member and the displacement of fluid confined in the apparatus. The apparatus to which the present invention pertains may be, but not limited to, an engine mount for a motor vehicle.
In a motor vehicle, for example, vibrations which greatly differ in frequency and amplitude from each other are developed depending upon the operating conditions of the vehicle, e.g. engine speed. A motor vehicle, therefore, should be furnished with a vibration preventing apparatus capable of absorbing a wide range of vibrations.
Vibration preventing apparatus heretofore proposed include one which uses fluid. Specifically, a fluid type vibration preventing apparatus is provided with a main fluid chamber at least partly defined by an elastic member such as a cushion rubber and variable in volume responsive to vibrations of a vibrating body, and an auxiliary fluid chamber communicated to the main fluid chamber through an orifice. Non-compressible fluid such as operating oil is confined in the main and auxiliary fluid chambers. The volume of the auxiliary fluid chamber is easily variable as the fluid flows into and out of the main fluid chamber due to changes in the volume of the same chamber.
The above-described type of vibration preventing apparatus absorbs vibrations having high frequencies and small amplitudes due to the elastic deformation of the elastic member, and damps vibrations having low frequencies and large amplitudes due to the resistance to the fluid which flows through the orifice. Consequently, vibrations of different frequencies and amplitudes are absorbed.
A problem with the fluid type vibration preventing apparatus is that it cannot absorb a sufficiently wide range of vibrations so long as the orifice is implemented with a single fixed orifice. Specifically, with regard to vibrations of high frequencies and small amplitudes, they cannot be effectively absorbed unless an orifice having a large diameter is provided to reduce the resistance to the flow of fluid and, thereby, the dynamic spring constant. On the other hand, with regard to vibrations of low frequencies and large amplitudes, what is necessary for them to be effectively absorbed is providing an orifice having a small diameter to increase the resistance to the flow of fluid and, thereby, the damping force. A single fixed orifice is incapable of settling such a dilemmatic situation, limiting the range of absorbable vibrations.
In the light of the above, there has been proposed a fluid type vibration preventing apparatus in which a main and an auxiliary fluid chamber are communicated to each other by two orifices which apply different degrees of resistance, as disclosed in Japanese Patent Laid-Open Publication (Kokai) No. 60-113832/1985. One of the two orifices exerting smaller resistance than the other is controlled to open and close depending on the operating conditions of a motor vehicle. Such an apparatus is elaborated for application to a vehicle engine mount and others which have to absorb a wide range of vibrations.
However, in a motor vehicle, for example, there occur a shake and other vibrations having considerably great amplitudes in addition to the previously stated high-frequency small-amplitude vibrations particular to a high-speed engine operating condition and low-frequency large-amplitude vibrations particular to an idling condition. It is desirable, therefore, that an engine mount for a motor vehicle be capable of absorbing all of such vibrations. The prior art apparatus having two different kinds of orifices fails to cope with all kinds of vibrations as stated above.
Another prior art fluid type vibration preventing apparatus is provided with a plurality of identical orifices which are opened and closed independently of each other, as disclosed in Japanese Patent Laid-Open Publication (Kokai) No. 60-113834/1985. In this kind of apparatus, the flow rate of fluid being moved between fluid chambers is changed by controlling the number of orifices which are open. However, the effect attainable with such a scheme is nothing less than the one attainable with a single orifice whose effective sectional area is variable, i.e., it is difficult to accommodate vibrations of various natures.